Audio Problems: Sounds fine in the edit, clicks and pops once exported.

Hi everyone. Apologies in advance if this has been addressed somewhere else but I am extremely new to FCP and am having difficulties finding information on this specific problem.

I am working with a combination of found footage from youtube with its original soundtrack and have added mp3 sound effects from FCPX's sound effects bank.

There does not appear to be an issue until I export it, when all of a sudden there will be a steady, repetitive clicking or popping audio distortion. I thought it may be a problem with audio clipping but I tried lowering the levels right down and this made no difference.

I suspect the issue might be related to the rendering but I am not very savvy in this department and am not sure how to approach it.

Any help or advice is welcome.

Regards,
The Red Herring

Well over a year and no response. And in my case this problem drives me nuts. I have about 30 minutes worth of a project that took almost hundred hours to complete (not counting tens of crashes of the FCPX) and once finished, the exported (damn "sharing") file is just useless due to pops and clicks scattered all over the final cut. No, the original audio is not MP3. No, it is not AAC either. No, converting and resyncing the audio does not fix the problem. No, there were no different samplerate audio clips combined. No, the audio levels are not too high. Etc, etc. Madness!